1. Field of the Invention
This invention relates to a system for controlling an air gap between a truck-carried element and a ground-mounted element of a linear motor which is used to propel a rolling stock of a railway or the like.
2. Description of the Prior Art
Various air gap controlling systems for a linear motor are already known, and an exemplary one of such conventional air gap controlling systems is disclosed, for example, in Japanese Patent Laid-Open Application No. 59-164264. The air gap controlling system is constructed such that a truck-carried element of a linear motor is carried directly on an axle which is mounted for yawing displacement on a truck. With the air gap controlling system, the air gap between a truck-carried element carried directly on an axle and a ground-mounted element mounted securely on the ground cannot readily be reduced to a value lower than 12 mm in a so-called standard condition, and accordingly, the efficiency of the linear motor is low at 55% or so with an air gap of 12 mm.
As factors which compel assurance of a large air gap between a truck-carried element and a ground-mounted element of a linear motor, there are, on the side of a truck-carried element, a tolerance in mounting of the truck-carried element, abrasion of a wheel and so forth, and on the side of a ground-mounted element, a tolerance in mounting of the ground-mounted element, a margin to maintenance recursion and so forth, and it is considered that, in order to prevent the truck-carried element and the ground-mounted element from colliding with each other in spite of those factors, an air gap of 6 to 7 mm must be assured.
Furthermore, if power is supplied to a truck-carried element and a ground-mounted element which constitute a linear motor, then an attracting force is produced between the truck-carried element and the ground-mounted element, and deflection takes place in both of the truck-carried element and the ground-mounted element due to such attracting force, vibrations and so forth. The amount of deformation of such deflection is normally about 2 mm. Further about 2 mm is required for a sink of a rail, a drop at a joint of rails and so forth, and besides further about 2 mm is required as a minimum margin. Such air gap must be increased as the running speed of the rolling stock is raised.
Accordingly, with the conventional air gap controlling system described above, it is difficult to make the air gap between a truck-carried element and a ground-mounted element smaller than 12 mm, and consequently, the efficiency of the linear motor is low.
Another exemplary one of conventional air gap controlling systems is disclosed in Japanese Patent Laid-Open Application No. 57-68608. According to the air gap controlling system, an air gap detector having a contact element for contacting with a ground-mounted element is provided on a truck-carried element carried on a truck, and a voltage from the air gap detector indicative of an air gap and a reference voltage from a reference voltage generator are supplied to a comparator. An output of the comparator is amplified and supplied to a servomotor, by which a hydraulic directional control valve is driven to supply pressure coil from a hydraulic oil source to a double acting hydraulic cylinder provided between a side beam of the truck and the truck-carried element so that the double acting hydraulic cylinder is operated to displace the truck-carried element upwardly or downwardly to control the air gap between the truck-carried element and the ground-mounted element. According to such conventional air gap controlling system, the servomotor is operated only in response to information obtained from the air gap detector and operates the hydraulic cylinder to control the air gap between the truck-carried element and the ground-mounted element.
With such conventional air gap controlling system, since the air gap detector is provided at the same location as the truck-carried element, even if the responding speeds of the servomotor, double acting hydraulic cylinder and so forth are high, it is difficult to control the air gap sufficiently due to a delay in response of the air gap detector because the speed of the rolling stock is high with respect to a sink of a rail, a drop at a junction between rails and an offset between ground-mounted elements. Besides, since the inertia of the truck-carried element is high, a rise of the responding speed of the truck-carried element results in increase in size of the construction and waste of energy. Further, deflection takes place in the truck-carried element and/or the ground-mounted element due to an attracting force or vibrations which are produced when power is supplied to the ground-mounted element and the truck-carried element, and it is difficult to achieve an appropriate air gap in spite of such variation in air gap because the air gap detector is mounted on the ground-mounted element. Accordingly, with such conventional air gap controlling system as described above, the amount of decrease in air gap by air gap control is about 6 mm.
A ground-mounted element which constitutes a linear motor is not installed at a branching location of rails. Accordingly, with the conventional air gap controlling system described above, the air gap detector may detect a surface of the ground below a ground-mounted element, a rail extending across and between a pair of rails or the like as a ground-mounted element in error. If such erroneous detection takes place, then the truck-carried element will be displaced downwardly by the hydraulic cylinder. If the truck runs after than at a location where a ground-mounted element is installed on the ground while the truck-carried element is held at the downwardly displaced position, then the truck-carried element may collide with and be broken by or break the ground-mounted element.